1. Field of the Invention
The present invention relates to a magneto-resistance effect element and a magnetic memory.
2. Related Art
It has been proposed that a magneto-resistance effect element using a magnetic material film is used not only for a magnetic head, a magnetic sensor or the like but also for a solid magnetic memory (magnetic random access memory or MRAM).
In recent years, a so-called “tunneling magneto-resistance effect element (TMR) has been proposed as a magneto-resistance effect element which has a sandwich structure film including two magnetic metal layers and one dielectric layer between positioned therebetween and which utilizes a tunnel current obtained by causing a current to flow perpendicularly to a film face of the structure film. Since the TMR element can achieve a magneto-resistance change ratio (MR ratio) of 20% or more even at room temperature, there has been growing technical developments for civilian applications to the MRAM element.
The TMR element can be realized by, after forming a thin Al (aluminum) layer with a thickness of 0.6 nm to 2.0 nm on a ferromagnetic layer, exposing a surface thereof to oxygen glow discharging or oxygen gas to form a tunnel barrier layer made of Al2O3. Recently, research and development for a TMR element using MgO (magnesium oxide) for the tunnel barrier layer have been conducted.
A ferromagnetic single tunnel junction element having a structure where an anti-ferromagnetic layer is given to one ferromagnetic layer of ferromagnetic layers sandwiching a tunnel barrier layer in ferromagnetic single tunnel junction so that the one ferromagnetic layer is utilized as a magnetization pinned layer whose magnetization has been pinned has been proposed. A ferromagnetic tunnel junction element including magnetic particles dispersed in dielectric and a ferromagnetic double tunnel junction element where a ferromagnetic film is a continuous film have been proposed.
There is a possibility of application to the MRAM even in these magneto-resistance effect elements, because the magneto-resistance change ratio of 20% to 220% has been achieved and decrease in magneto-resistance change ratio can be suppressed even when a value of a voltage applied to a TMR element is increased in order to obtain a desired output voltage value. When the TMR element is used as a memory element for a MRAM, one of ferromagnetic layers sandwiching a tunnel barrier layer is formed as a magnetization pinned layer and the other thereof is formed as a magnetic recording layer (a magnetization free layer). A memory element using the ferromagnetic single tunnel junction or the ferromagnetic double tunnel junction is non-volatile, and has such a potential that a writing/reading time is reduced to 10 nanoseconds or less and the number of rewriting times is 1015 or more.
Regarding a cell size of a memory, however, when an architecture where each memory cell includes a transistor and a TMR element is used, there is such a problem that a size of the cell can not be reduced to that of a DRAM (dynamic random access memory) made of semiconductor or less.
In order to solve the problem, a diode type architecture where a TMR element and a diode are connected in series between a bit line and a word line and a simple matrix type architecture where a TMR element is arranged between a bit line and a word line have been proposed.
In both the cases, however, since inversion of magnetization of the magnetic recording layer is performed by current magnetic filed due to current pulses at a time of writing in a magnetic recording layer, there is such a problem that power consumption is large, mass storage can not be achieved due to an allowable current density limit in writing for achieving the mass storage, a driver area for allowing current to flow becomes large.
In order to overcome the above problem, there has been proposed a solid magnetic memory where a thin film made of magnetic material with a high magnetic permeability is provided around a writing wire. According to the magnetic memory, since the magnetic film with a high magnetic permeability is provided around the writing wire, a value of a current required for information or data writing into a magnetic recording layer can be reduced efficiently. Even if such a magnetic memory is used, it is much difficult to reduce a writing current to 1 mA or less.
In order to solve these problems, a writing method utilizing a spin injection process has been proposed. This spin injection process is configured to utilize inversion of magnetization of a magnetic recording layer in the memory element performed by injecting spin-polarized current into the magnetic recording layer. In the spin-injection process, when magnetization of the magnetic recording layer is inverted by injecting spin-polarized current, when an area of a memory element is large, annular magnetic field due to the current is generated, so that inversion of magnetization does not take place.
Further, since high integration is required for application as a solid magnetic memory, it is necessary to form an element with a small area and with reduced fluctuation in area. Further, it is necessary to allow low current writing and provide a thermal stability of a magnetic recording layer to a thermal fluctuation even in cells with various sizes including a small cell with a size of 0.1×0.1 μm2 which is a limit in thermal fluctuation tolerance.
In the conventional TMR element, however, since a current density required for spin-injection writing is high such as 107 A/cm2 or so, such a problem arises that a tunnel barrier layer may be destroyed. Accordingly, in order to realize stable writing without causing TMR element destruction, reduction in writing current is required. In application to the MRAM, reduction in writing current results in reduction in power consumption, the reduction in writing current is an essential technique. Incidentally, for a giant magneto-resistance (GMR) effect element, a dual pin structure has been proposed as an approach for reducing writing current by one digit without depending on a direction of the current. In the dual pin structure, orientations of magnetizations of first and second pin layers are anti-parallel to each other and a non-magnetic layer and a second pin layer constituting a spin reflection layer are respectively made of Cu and Co90Fe10 so that it is made possible to reduce inverted current densities for magnetization inversion due to spin injection to 6×106 A/cm2 or so (see 49th MMM conference digest HA-05, for example). However, further reduction in current density is required for memory application.
As explained above, the current density required for spin injection writing is as high as 6×106 A/cm2 or so in the conventional tunnel magneto-resistance (TMR) effect element, which leaves the problem about the reduction in current density.